There is a group of consistency freaks out there (like myself) who are not very fond of mixing Gtk+ with Qt applications. The reasons for this are not just graphical, but also behavioural; Gtk+ applications behave differently than Qt applications (menus are different, layout is different, etc.). Trolltech obviously can't tackle the behavioural side of this issue, but they can tackle the graphical one. This is exactly where QGtkStyle comes into play.

Gtk developers are thinking of trying to catch up to Qt, that's why they are deliberating a GTK 3.x branch... But it'll take them years, and Qt will be further developed by then. So catching up? Not anytime soon, I suppose. But of course, it also depends on what you want and need in a toolkit...